In this paper, we investigate sensible and latent heat transfer through heat exchanger matrix structures containing phase change material (PCM) in the interstitial spacing. The heat transfer is driven by a temperature difference between fluid flow passages and the phase change material matrix which experiences sensible heat transfer until it reaches the phase change material fusion point; then it undergoes melting or solidification in order to store, or reject, energy. In prior work, a dimensionless framework was established to model heat transfer in a thermal energy storage (TES) device much like effectiveness-NTU analysis methods for compact heat exchangers. A key difference, however, is that in TES units, the overall heat transfer coefficient, U, within the phase change material matrix varies spatially in the unit and with time during storage or extraction. Determination of a mean U for these processes is a key challenge to applying the effectiveness-NTU analysis to design of a TES unit. This paper assesses and identifies strategies for determining the matrix overall heat transfer coefficient in a TES unit from model predictions or experiments. The sensitivity of the TES energy efficiency to the matrix overall heat transfer coefficient is also explored, and the implications for some typical applications are discussed.

In this work, we focus exclusively on heat transfer enhancement techniques for the air-side heat transfer in air-cooled heat exchangers/condensers. An innovative dimpled fin configuration is explored. Experiments, in which both heat transfer and drag are measured, are conducted with flat tubes in three configurations: without fins, with plain fins and with dimpled fins. Reynolds numbers based on the hydraulic diameter of the finned passages are varied between 600 and 7000. Results indicate that fins are more advantageous at lower Reynolds numbers since the increase in drag at higher Reynolds numbers quickly erases any advantage due to an increase in heat transfer rate. As an example, for the plain fins versus a bare tube at a Reynolds number of 600, there is a 7 fold increase in heat transfer with only a 5 fold increase in drag. However, at a Reynolds number of 7000, both heat transfer and drag increase by approximately 6 times, indicating that the increase in drag has caught up with the heat transfer enhancement. Similarly, while dimpled fins do result in higher heat transfer compared with the plain fins, the advantage is also more prominent at lower Reynolds numbers where heat transfer enhancement is higher than the associated increase in pumping power.

This work experimentally studied the convective heat transfer characteristics of a novel nanostructured heat transfer fluid: “Ethanol/Polyalphaolefin(PAO) nanoemulsion fluids” flowing through a heat exchanger made of twelve circular minichannels. Ethanol/PAO nanoemulsion fluid is a thermodynamically stable system formed by dispersing ethanol into a mixture of PAO and surfactants, in which the ethanol added inside forms self-assembled nanodroplets of tens of nanometers in diameter. These ethanol nanodroplets can serve as pre-seed boiling nuclei at elevated temperature. The Reynolds number was varied between 140 and 1200 to maintain the entire range of flow regime remained at laminar flow for both single- and two-phase convective heat transfer experiments. Pure PAO was also tested under same conditions and used as baseline data for comparison. It is found that: for single phase flow, there is no significant increase in Nusselt number of Ethanol/PAO nanoemulsion compared to that of PAO fluid in laminar flow regime. However, when the nucleation of ethanol nanodroplets inside the nanoemulsion fluid was initiated, it showed a substantial increase in heat transfer coefficient compared to that of PAO fluid: a 75% enhancement can be achieved under current test conditions. While its mechanism is not completely clear yet, it is believed that such an effect is likely related to the latent heat carried by ethanol bubbles, as well as the increased turbulence and mixing generated during the two-phase flow of nanoemulsion which can increase the heat transfer rate.

This paper discusses the design, analysis, and testing of a Water Cooling System (WCS) for a Drift Tube Linear (DTL) Particle Accelerator structure at the Los Alamos Neutron Science Center (LANSCE). The DTL WCS removes large amounts of dissipated electrical energy in a very controlled manner to maintain a constant temperature of the large structure. First, the design concept and method of water temperature control is discussed. Second, the layout of the water cooling system, including the selection of plumbing components and instrumentation is presented. Next, the development of a numerical nodal network model, used to size the plumbing, pump, control valves, and mixing tank (heat exchanger), is discussed. Finally, empirical pressure, flow rate, and temperature data from a functioning DTL water cooling system are used to assess the water cooling system performance and validate the numerical model.

Heat transfer is a naturally occurring phenomenon which can be greatly enhanced by introducing longitudinal vortex generators (VGs). As the longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty, VGs are widely used to enhance the heat transfer of flat-plate type heat exchangers. However, there are few researches which deal with its thermal optimization. Three dimensional numerical simulations are performed to study the effect of angle of attack and attach angle (angle between VG and wall) of vortex generator on the fluid flow and heat transfer characteristics of a flat-plate channel. The flow is assumed as steady state, incompressible and laminar within the range of studied Reynolds numbers ( Re = 380, 760, 1140). In the present work, the average and local Nusselt number and pressure drop are investigated for Rectangular vortex generator (RVG) with varying angle of attack and attach angle. The numerical results indicate that the heat transfer and pressure drop increases with increasing the angle of attack to a certain range and then decreases with increasing angle of attack. Moreover, the attach angle also plays an importance role; a 90° attach angle is not necessary for enhancing the heat transfer. Usually, heat transfer enhancement is achieved at the expense of pressure drop penalty. To find the optimal position of vortex generator to obtain maximum heat transfer and minimum pressure drop, the data obtained from numerical simulations are used to train a BRANN (Bayesian-regularized artificial neural network). This in turn is used to drive multi-objective genetic algorithm (MOGA) to find the optimal parameters of VGs in the form of Pareto front. The optimal values of these parameters are finally presented.

A novel design of a mini heat exchanger utilizing forced convection heat transfer enhancement with electrohydro-dynamic (EHD) technique has been numerically investigated. When a high voltage is applied to a metal wire, air in its vicinity will be ionized and the injected ions will travel towards electrically grounded heat exchanger surfaces, leading to the corona wind. As a result, the corona wind disturbs the heat exchanger boundary layer and thus enhances heat transfer between the heat exchanger surface and its ambient air. A three dimensional numerical model has been developed to evaluate the heat transfer coefficient (HTC) and air side pressure drop of this EHD enhanced mini heat exchanger. Influences of position and size of the wire are evaluated in order to achieve the highest enhancement. In addition, the swirling flow pattern induced by EHD has been studied due to its important role in heat transfer enhancement. The results show a three times increase of HTC enhanced by EHD effect in present design comparing to the one without EHD effect. The most promising result shows an overall heat transfer coefficient equal to 318 W/(m 2 ·K) for a bare tube in cross flow configuration with airside pressure drop of 2.8 Pa.

A methodology using Computational Fluid Dynamics (CFD) was developed to predict the flow and heat transfer performance of a single two dimensional sinusoidal channel of a Heat Exchanger (HE) at a Reynolds number (Re) range of 5 ≤ Re ≤ 500. The impact of different modelling assumptions was thoroughly evaluated which has not has been done in detail before. Two computational domains were used: a single period sinusoidal channel for fully periodic flow predictions and finite length channel consisting of 6 sinusoidal channel periods. Mesh and time independence was achieved for both domains whilst results with periodic domain were compared to numerical results in the literature. Laminar, k-ε and k-ω SST predictions were assessed throughout the Reynolds range with unsteady flow onset detected at Re ≈ 200 using laminar and k-ω SST models. The impact of different accuracy numerical discretisation schemes is assessed throughout the Re range and it was found that second order accuracy schemes should be used to fully capture the unsteady flow. Comparison between open-source CFD package OpenFOAM and Ansys was Fluent was performed and agreement was ‘ found.

The exponential growth in electronic power has brought amazing technology but with it also comes a burden in high heat production that threatens the safety of the product since electronics’ failure rate increases by its operating temperature. As such, cooling techniques have a key role to keep the temperature of electronics devices, such as processors, memory and graphics chips, below a maximum operating temperature. In this work, a porous filled heat exchanger has been numerically modeled to investigate the cooling effectiveness and temperature distribution on the base of the heat exchanger subjected to high heat flux leaving these devices. The effects of different nanofluid coolants (0.75% double walled carbon nanotube in water (DWCNT), 1% alumina in water, and 1% diamond in 20:80 ethylene glycol/water), porous materials (copper and annealed pyrolytic graphite (APG)), and porosity values are investigated. The coolant enters from an inlet channel normal to the base, moves through the porous field, and then leaves the heat exchanger through two opposite exit channels parallel to the base. The study is performed for two and three dimensional geometries in which two different designs are studied for 3D cases. One of the designs has a rectangular cross sectional inlet channel (along transverse direction) and the other design has square one. The results indicate that utilizing APG porous matrix, for all studied coolants of pure water and water based nano fluidics, improves substantially the cooling of the base of the heat exchanger in 2D and 3D with rectangular inlet. The results also show that utilizing carbon nanofluids (DWCNT) as coolant for high porosity structures, in both copper and APG porous matrices, improves cooling efficiency and temperature uniformity over the base, for all 2D and 3D cases. The effect of inlet channel geometry, square and rectangular, is also investigated for either similar velocity or similar mass flow rate at the inlet channel entrances.

In this work for the first time the performance of multi-stage shell and tube Transport Membrane Condenser (TMC) based heat exchangers are evaluated numerically. The present heat exchanger is design to work under high pressure and temperature condition for both heat and water recovery in Oxy-Combustion processes. TMC heat exchangers use the nano-porous and ceramic membrane technology to extract the water vapor and latent heat of condensation from the flue-gas. The most important application of TMC heat exchangers is in the power plants which the water vapor in the presence of other non-condensable gases (i.e. CO2, O2 and N2) exist. Effect of the different arrangement of the multi-stage shell and tube TMC heat exchangers, number of branches and number of heat exchangers in each branch on the heat transfer and water recovery have been studied numerically. A single phase multi-component model is used to assess the capability of single stage TMC heat exchangers in terms of waste heat and water recovery at various inlet conditions. Numerical simulation has been performed using ANSYS-FLUENT software and the condensation rate model has been implemented applying User Define Function. Finally, an optimum configuration for the TMC heat exchanger unit has been proposed and the results of numerical simulations are depicted in terms of temperature and water vapor mass fraction contours.

Experimental validation was performed in this study to verify the efficacy of numerical models for predicting the location of solid-liquid interface in an axi-symmetric configuration during both melting and solidification in a Latent Heat Storage Unit (LHSU). Development of analytical solutions for predicting the location of the solid-liquid interface is often intractable in LHSU due to non-linear temperature distribution in the Phase Change Material (PCM). This is further complicated by the moving boundary problem with free convection within the liquid phase of the PCM. Analytical solutions available in the contemporary literature are based on simplified transient heat conduction models and often fail to reliably predict the charging and discharging time constants for LHSU with complex configurations. This study is designed with the goal of developing more sophisticated numerical models for the estimation of transient thermal performance of an LHSU with a simple configuration involving a shell and tube heat exchanger (HX). The LHSU utilized in this study is realized by integrating various types of Phase Change Materials (PCM) contained in the shell side of a HX. The LHSU is charged or discharged by pumping hot or cold fluids in the tube side of the HX (i.e., by pumping water at a fixed inlet temperature from a commercial chiller apparatus). This study enabled the characterization of the transient response of a LHSU subjected to conduction and forced convection heat transfer. The PCM used in this material was paraffin wax (PURETEMP 29). The HX in the LHSU consisted of a single pass straight tube (½ inch copper pipe) mounted within a single shell configuration. The shell was fabricated from plastic material using additive manufacturing (i.e., “3D Printing”). The temperature variation during melting and solidification of the PCM were measured at different radial and axial locations within the cylindrical shell that was mounted vertically. Temperature measurements were performed at different mass flowrate ranging from 0.004 Kg/sec to 0.007 Kg/sec for the same fluid temperature. The water bath temperatures were maintained at a constant temperature of 40°C for melting and 15°C for solidification. The experiment results show that the transient response of the LHSU for charging and discharging (i.e., time required for melting and solidification of the PCM) vary significantly. Comparison of the experimental data with analytical results (involving quasi-stationary models for phase change) demonstrate that natural convection is the dominant mode during the melting process, while conduction is the dominant mode during the solidification process.

An experimental study on local flow boiling heat transfer performance in two longitudinal dimple-grooved tubes and an equivalent smooth tube was performed. All three test cooper tubes have the same inner diameter of 11.5 mm; the working fluid is the near-azeotropic mixture, R410A; and all test runs are conducted in a 2 m long horizontal tube-in-tube heat exchanger. Constant evaporation temperature at 10 °C was maintained when heat flux ranging from 32 kW/m 2 to 37 kW/m 2 and refrigerant quality varied from inlet 0.1 to outlet 0.9 at mass flux 150 kg/(m 2 s). The local heat transfer coefficients were obtained for all test conditions using refrigerant R410A. The test results for evaporation were presented compared to the equivalent smooth tube. Wall temperature and local and average heat flux is measured; heat flux effect and surface superheat effect is discussed on the tube side evaporation. The enhanced heat transfer area of two longitudinal dimple-grooved tubes are 1.02 and 1.03, respectively.

In the wake of utilization of novel materials in various thermal applications open cell metal foams have received attention due to their inherent properties such as large surface area to volume ratio and higher thermal conductivity. Additionally, complex tetradecahedron structure promotes mixing and makes them a good candidate for heat transfer applications. In this paper, a relative comparison has been made between the thermal-hydraulic performance of aluminum and copper metal foam heat exchangers with the same geometry under dry and wet operating conditions. Heat exchanger consisting of round tube with annular layer of metal foam have been considered. Experiments have been conducted using a closed-loop wind tunnel to measure the heat transfer performance and pressure drop. The impact of base metal (aluminum and copper) on the heat transfer rate has been evaluated at varying air flow rates and upstream relative humidity. It has been found that due to similar geometry (flow depth, face area, pore size) both aluminum and copper foam samples have comparable pressure drop under dry conditions. However, the pressure gradient was noticeably different for two samples under wet operating conditions. An obvious difference in heat transfer rate for aluminum and copper metal foam heat exchangers was observed under both dry and wet operating conditions. The findings have been explained in terms of the impact of the thermal conductivity of base metal and condensate retention.

The rapid development of Additive Manufacturing (AM) technologies has provided engineers with new methods to design and fabricate complex mechanisms. AM offers unique methods to allow for integration and simplification of components, reduced manufacturing time, fabrication of complex-shaped objects, improvements upon existing designs, and extending the creative design space which engineers rely on for ingenuity. For many applications, heat exchanger performance can be improved by reducing its size, increasing the overall heat transfer coefficient and surface area, and making more efficient use of the mechanical structure for heat exchange. Traditional manufacturing often limits or prohibits many of these enhancements due to increased manufacturing and assembly costs. This study explored using AM to design and fabricate a compact twisted tube stainless steel shell and tube heat exchanger that would improve upon all of the features just mentioned. This paper discusses the design of the heat exchanger and the AM technique used to fabricate a prototype. The manuscript will show via CFD analyses, how the heat transfer area of the unit was improved 18% and the overall heat transfer coefficient as increased by 40% over a traditional round tube heat exchanger with an identical footprint. Further, the study will show how AM was leveraged to combine five manufacturing steps into one to fabricate a prototype, fully functional twisted tube heat exchanger.

Plate fin heat exchanger is widely used for air side cooling. To enhance its thermal performance, a novel self-agitator for air-side heat transfer enhancement was developed and validated both numerically and experimentally. Self-Agitator made of 3D printing material connected to well-designed metal beam was placed between the plate fins. It could lose stability and start to oscillate in the channel due to fluid structure interaction. The oscillation enhanced the mixing so that self-agitator can improve the heat transfer in plate fin. Wind tunnel experiment was carried out and self-agitator can save pumping power up to 40% with the same rejected heat. Numerical model was also developed and verified for this fluid-structure interaction process.

Heat exchanger performance data commonly contain redundant heat transfer rate measurements. Due to measurement uncertainties involved in the experiments, these redundant heat transfer rates have some discrepancies. While it is a common practice and adopted by engineering standards to use the arithmetic mean of heat transfer measurements, the resulting performance indicators of heat exchangers do not result in a minimum uncertainty possible. Also, this approach fails to resolve discrepancies in resulting transport performance parameters depending on the use of UA-LMTD method or effectiveness-NTU method. In this paper, heat exchanger performance data with two heat transfer measurements from hot and cold fluid streams are combined to produce a least uncertainty of the performance indicators. Individual measurements of mass flow rates and temperatures are corrected by most likely errors based on their respective uncertainties. The validity of this method has been demonstrated by Monte-Carlo simulations. Using air conditioning heat exchanger performance data under dry and wet surface conditions, it is demonstrated that the proposed method leads to a minimum uncertainty of the calculated variables.

Additive manufacturing, the layer-by-layer creation of parts, was initially used for rapid prototyping of new designs. Recently, due to the decrease in the cost and increase in the resolution and strength of additively manufactured parts, additive manufacturing is increasingly being used for production of parts for end-use applications. Fused Deposition Modeling (FDM), a type of 3d printing, is a process of additive manufacturing in which a molten thermoplastic material is extruded to create the desired geometry. Many potential heat transfer applications of 3d printed parts, including the development of additively manufactured heat exchangers, exist. In addition, the availability of metal/polymer composite filaments, first used for applications such as tooling for injection molding applications and to improve wear resistance, could lead to increased performance 3d printed heat exchangers because of the higher thermal conductivity of the material. However, the exploitation of 3d printing for heat transfer applications is hindered by a lack of reliable thermal conductivity data for as-printed materials, which typically include significant void fractions. In this experimental study, an apparatus to measure the effective thermal conductivity of 3d printed composite materials was designed and fabricated. Its ability to accurately measure the thermal conductivity of polymers was validated using a sample of acrylic, whose conductivity is well understood. Finally, the thermal conductivities of various 3d printed polymer, metal/polymer composite, and carbon/polymer composite filaments were measured and are reported in this paper. The materials used are acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), stainless steel/PLA, Brass/PLA, and Bronze/PLA.

An experimental investigation was performed to study the heat transfer and pressure drop characteristics of refrigerant R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) at low mass flux. The heat transfer coefficient and pressure drop characteristics are analyzed in relation to varying mass flux (30–50 kgm −2 s −1 ), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm −2 ), and vapor quality (0.1–0.9). The two-phase pressure drop shows a strong dependence on mass flux and significant saturation temperature drop at high mass flux. The two-phase heat transfer coefficient was both strongly dependent on heat flux (at vapor qualities below 0.4) and on mass flux (at vapor qualities above 0.4). There was also apparent dryout, as depicted by decreased heat transfer at high vapor qualities. These observations suggest that both nucleate and convective boiling mechanisms prevailed. Existing transition correlations however suggest that the experimental data is rather convection-dominant and not a mix of convection and nucleate boiling. The experimental data further strongly suggest the prevalence of both macrochannel and minichannel type flows. Several acknowledged semi-empirical transition criteria were employed to verify our observations. These criteria mostly support our observations that R-134a evaporating at low mass fluxes in a BPHE with a hydraulic diameter of 3.4 mm, has heat transfer and pressure drop characteristics typically indicative of macrochannel as well as minichannel flows. Disagreement however exists with accepted correlations regarding the prevalence of convective or nucleate boiling.

Stable homogeneous colloidal suspensions of nanoparticles in a liquid solvents are termed as nanofluids. In this review the results for the forced convection heat transfer of nanofluids are gleaned from the literature reports. This study attempts to evaluate the experimental data in the literature for the efficacy of employing nanofluids as heat transfer fluids (HTF) and for Thermal Energy Storage (TES). The efficacy of nanofluids for improving the performance of compact heat exchangers were also explored. In addition to thermal conductivity and specific heat capacity the rheological behavior of nanofluids also play a significant role for various applications. The material properties of nanofluids are highly sensitive to small variations in synthesis protocols. Hence the scope of this review encompassed various sub-topics including: synthesis protocols for nanofluids, materials characterization, thermo-physical properties (thermal conductivity, viscosity, specific heat capacity), pressure drop and heat transfer coefficients under forced convection conditions. The measured values of heat transfer coefficient of the nanofluids varies with testing configuration i.e. flow regime, boundary condition and geometry. Furthermore, a review of the reported results on the effects of particle concentration, size, temperature is presented in this study. A brief discussion on the pros and cons of various models in the literature is also performed — especially pertaining to the reports on the anomalous enhancement in heat transfer coefficient of nanofluids. Furthermore, the experimental data in the literature indicate that the enhancement observed in heat transfer coefficient is incongruous compared to the level of thermal conductivity enhancement obtained in these studies. Plausible explanations for this incongruous behavior is explored in this review. A brief discussion on the applicability of conventional single phase convection correlations based on Newtonian rheological models for predicting the heat transfer characteristics of the nanofluids is also explored in this review (especially considering that nanofluids often display non-Newtonian rheology). Validity of various correlations reported in the literature that were developed from experiments, is also explored in this review. These comparisons were performed as a function of various parameters, such as, for the same mass flow rate, Reynolds number, mass averaged velocity and pumping power.

Waste heat is a major energy loss in manufacturing facilities. Thermally conductive polymer composite heat exchangers could be utilized in the ultralow temperature range (below 200° C) for waste heat recovery. Fused deposition modeling (FDM), also known as three-dimensional (3-D) printing, has become an increasingly popular technology and presents one approach to fabrication of these exchangers. The primary challenge to the use of FDM is the low-conductivity of the materials themselves. This paper presents a study of a new polymer-Zn composite designed for enhanced thermal conductivity for usage in FDM systems. Thermal properties were assessed in addition to basic printability. Filler volume percentages were varied to study the effects on material properties. Scanning electron microscope (SEM) images were taken of the 3-D printed test pieces to determine filler orientation and filler distribution. Lastly, experimentally obtained thermal conductivity values were compared to the theoretical thermal conductivity values predicted from the Lewis-Nielsen model.

Manifold-microchannel combinations used on heat transfer surfaces have shown the potential for superior heat transfer performance to pressure drop ratio when compared to chevron type corrugations for plate heat exchangers (PHE) [1–4]. However, compared with heat transfer enhancements such as intermating troughs and Chevron corrugations, manifold-microchannels (MM) have several times more variables that influence the heat transfer and pressure drop characteristics, including microchannel width, depth, passes, manifold depth, width, and manifold fin thickness. Previous work has reported on the effects of some of the variables, and provides some models for their effects on thermal and hydraulic performance. The current paper presents a genetic algorithm (GA)-based procedure to analyze the implicit effects of some of the manifold-microchannel variables, and compare the performance of manifold-microchannel plate heat exchangers to those using standard Chevron corrugations. The objective of the present work is to evaluate the performance of manifold-microchannel heat transfer enhancements and demonstrate the potential for using GA-based procedure to optimize the heat exchanger. This paper also presents the modifications of the standard GA algorithm when applied to the optimization of MM. The resulting GA procedure is particularly well suited to PHEs for several reasons, including the fact that it does not require continuous variables or functional dependence on the design variables. In addition, the computational effort required for the GA technique in our implementation scales linearly, with a scaling coefficient that is significantly less than one, making it economical to analyze PHEs with several variables with degrees of freedom (DOF) with respect to the fitness function. The results of optimizing a manifold-microchannel plate heat exchanger are presented, and the exchanger’s performance is compared to more conventional PHE of the same volume utilizing chevron corrugations. Finally, results from the empirical procedure presented in this paper for a manifold-microchannel are compared with experimental measurements in Andhare [5].